When an airplane is landing on a gravel runway it normally touches down first with its main wheels, and shortly thereafter with its nose wheel. When a stationary wheel contacts the gravel it "plows" for a short time until it is being spun-up by the contact with the ground. When such plowing occurs, stones and dirt are often being thrown upwards. Now, if the airplane happens to be equipped with jet engines which are stradle mounted on the rear portion of the fuselage, there is a real danger that the debris raised by the nose wheel will be sucked by the engines, potentially damaging their internal parts.
It has been established that the amounts of debris raised by the nose wheel can be minimized by spinning-up the nose wheel prior to its touch-down.